Rapid Spin: Gets rid of entry hazards, something I don't use but that I sometimes allow my foes to be able to use.
Gyro Ball: Forretress has incredibly low Speed (mine has 76), especially with Macho Brace. It also has high accuracy and STAB, and can deal with Rock and Ice types.
Earthquake: Is Earthquake.
Zap Cannon: High power, always paralyzes when it hits, deals with Water and Flying types.

15 Answers

Forretress is famous for its spinning, and placing hazards, so make him a Suicide lead!

Stealth Rock (Open up with this. As much as I hate it, it's got its use)

Sandstorm / Gyro Ball / Counter (Set this up for more
damage. If you're team doesn't mix well with Sandstorm, try Counter. There are rarely Special-based leads on a team, because of their usual frail defense [there are exceptions of course], so Counter is Viable. If you're still not satisfied, then just try Gyro Ball)

Rapid Spin (Spin off whatever the opponent tries to lay down. Makes for a predictable, but nonetheless, effective Lead Counter)

Explosion (When the opponent has been spinned, if necessary, and you've placed down your Stealth Rock, sandstorm, hit with Gyro Ball, etc. Use Explosion for the guaranteed (99.99% of the time!) KO.

For me though, personally, Dual Screener / Anti-Lead mix is the best. It won't rip holes through a team (Who expects Forretress to do such a thing anyway, right?), but it provides incredible support. It's all Defensive, but I've never seen a Taunting lead before (Taunt-Steelix maybe, but how often do you see that?), so you should be safe. To make it appealing though, I can add some alternatives:

Light Screen (One screen.)

Reflect (Two screens.)

(Note that, if your team may not need one of the screens, then you can replace one of these with Gyro Ball, Bug Bite, etc.)

Rapid Spin (Spin off whatever the opponent tries to place down)

Swagger (Base 75 HP and Base 140 Defense? Why not!)

One more set. This one is more of a gimmicky, tricky set, but it still works great! It works similarly to Baton Pass Attacking Shuckle, except it's easier to utilize. You don't NEED to Baton pass any Defense to Forretress, but it saves time, is all. Here it is, folks. Forretress' newest set addition from Gen IV: A PHYSICAL SWEEPER SET. =D

Payback / Rock Slide / Earthquake (One flinches and has coverage, the other is strong and has coverage, and the last one is just...Godly. Pick your poison)

WOW, it's been a while since I wrote something this long. xD

Anyway yeah, these are my recommendations. The above sets are good too, but I always found Forretress too Wall-like to just go ahead and sweep, like in the above answers. It's fine for In-Game, but he'd get eaten alive on a competitive-scale, and even on a Battle Tower-scale.

Earthquake - as you said
Zap Cannon - paralyzed the foe if hits
and the other (mine moveset)
Toxic - annoying badly poison
and Explosion - I don't know, but if you low on HP and don't know what to do, use it (but don't use it against wild pokemon, you know why, didn't get exp)
that's my moveset...
or you can keep gyro ball and don't use my explosion!

I have 2 Forretress in my platinum, one male and one female, the moveset above is male one. The female moveset and strategy (usually use in wireless battle against higher level) are
Stealth Rock, after that I use Toxic Spikes, and then Spikes, and you must be low on HP, right? Just use Explosion... lol xD
and don't use this moveset, it's terrible. I use it just for beat my friend's Mewtwo lv80...(all of my pokemon lv70)

Earthquake-covers only weakness
Gyro ball-Works well with speed. STAB
Light Screen-Covers his special defense
Toxic or toxic spikes-poisons foe

OR:

Earthquake-just like you said
Gyro ball-look above
Zap cannon
stealth rock/toxic spikes/RS/spikes-Only ONE!! I hate when people use all 3, like wobbuffet said, it looks good on the surface, but is terrible in application.

This probably one of the strangest Forretress sets out there, and for a good reason. Counter + Gyro Ball Forretress takes a more offensive approach at this bagworm, providing hazards, spinning your opponents' out and dealing damage. Using Counter means you need to keep your prediction skills sharp; it isn't easy to pull off, but it is very rewarding if you can. If you catch a Dragon going for Outrage, for example, you can use Counter to deal massive damage while the opponent is locked into the move. And with Sturdy, Forretress can switch into something with some ridiculous Attack boost and OHKO it with Counter.

Rapid Spin is Forretress' claim to fame; remove your opponent's hazards, which helps your team a lot. The last moveslot is your way of getting entry hazards, choose whatever one. Gyro Ball is for fast sweepers who won't appreciate a hit. I know Choice Scarf Gengar is OHKO'd by it, which is pretty stupid really.

The best teammate for this guy is any user of Wish. This allows it to heal and take more damage, and stick around for longer to spin away hazards. Blissey/Chansey are probably the best users of this. This pretty much recreates SkarmBliss, although that isn't a bad thing.

This kind of Forretress fits perfectly with teams focused on HO, which love the resistances and support it provides.

A wicked set that I like using in Ubers especially to help take down Kyogre. Spikes (Or Rocks if preferred) setter and Spinner with a twist. Pain Split + Sturdy allows me to stay in on any strong special attack, live, and suck them dry before switching out. I once got a rage quit on the first turn doing it (xD).

i like this idea but im not much for spikes i would give him earthquake instead. Also i would give him 252 att/252h hp to make his att ass high as i could and the maxed out hp would support that awesome 140 def really well

Need a Spinner/Rock Setter that is exceptionally bulky and can live for a great deal of long, drawn out battles? ChestoRest Forretress is your man. One of the most reliable Rock Setters and Spinner in OU, Gyro Ball provides STAB and hits faster Pokemon, such as Dragonite, Landorus, and even OHKO'ing most Terrakion. Rest is the crux, using his Chesto Berry to effectively shrug off status, completely replenish his HP, and refresh Sturdy. Also, Forretress's abysal Speed allows him to move last every time, allowing Rest to heal the most HP, and guarantee that Sturdy is not broken by the opponent moving last.

I have used this set countless times. He is able to lay down hazards, spin them away, and get sweepers in for free. The item depends on what your team needs, but I find that leftovers works best in OU and custap works best in ubers.

The point of this moveset is to make sure that opponents switching in take a decent amount of damage. Stealth Rock should be placed first to ensure good damage to flying types, Toxic spikes should be placed second so the opponent is sure to be intoxicated, and spikes go last, to add additional damage. Rapid Spin is there to clear out any traps laid by the opponent, but you could give Forretress Explosion if you think there would be no further point in having him in battle